WRT54G, very slow browsing and creating connections

I have five computers connected to a WRT54G router. Two laptops, both through WLAN and three PCs of which one is wired to the router. I'm using the latest official firmware. My connection (to the Internet) is 10 Mb (ten megabits).

There's something going on with the router...when the wired PC is the only one online (or powered on) everything works fine. But when another computer is powered on the network, surfing and pretty much everything slows down even the admin pages for the router which I find very surprising. Also, when software tries to establish connections it seems to take forever (10-20 seconds, not all of the time of course) or then it doesn't work at all.

The wired computer uses eDonkey most of time, as does at least one of the other computers. I'm starting to think that the problem is because of eDonkey and other P2P apps running on two or more computers simultaneously. How many simultaneous connections can the WRT54G handle? Or does something else cause the problem? As I've found out, at least HyperWRT would increase the amount of connections. Could this help?